The present invention relates to electric vehicles and, more particularly, to controls for electrically powered vehicles having differential drive and steering systems. In accordance with the present invention, electrically powered vehicles of improved handling, maneuverability, and overall driving characteristics are provided. The improvements of the present invention are especially advantageous when incorporated in small personal vehicle carriers such as electric wheelchairs, since the improved smoothness of operation and stability are of special importance to operators or vehicle drivers who are physically impaired.
The prior art controls for electrically powered wheelchairs and the like display undesirable operational characteristics, including abrupt movement in operation or "jerkiness", and tend to require an undue amount of effort to establish or regain directional control in dynamic situations. To a large degree, the deficiencies of the prior art controls are believed to be based upon a failure to recognize the unique operating characteristics and control requirements of differentially powered vehicles.
The vehicles of concern herein typically include left and right drive wheels which cooperate with one or more nondriven wheels to fully support the vehicle in a stable condition. The nondriven wheels may be free-turning caster wheels. Each drive wheel is independently driven in both speed and direction by an associated direct current electric drive motor so that the vehicle can be caused to move forward or backwards, or to turn right or left.
The steering function is provided by controlling the relative speed and direction of rotation of the drive wheels. If both drive wheels are operated at the same speed and rotated in the same direction, the vehicle travels in a straight line. The operation of the wheels at different speeds achieves turning of the vehicle, with the higher speed wheel tracking along the outer circumference of the path of the turning vehicle and the lower speed wheel tracking along the inside circumference of the path.
The relative speed and direction of rotation of the wheels are regulated by controlling the associated drive motors. To that end, motor speed is controlled by varying the voltage applied to the motor and motor direction is changed by reversing the polarity of the applied voltage. The prior art provides two methods of control, which are referred to herein as Alpha and Beta as defined below and subsequently contrasted with the novel Gamma method of the present invention.
The Alpha method comprises a relatively simple technique to achieve steering control and to effect relative speed differences between the drive wheels. The Alpha method adopts the simple and direct expedient of allowing the inside drive wheel to slow down by removing or reducing the applied voltage to the associated inside drive motor. Friction will gradually cause the inside motor to go slower, and the vehicle will turn. The disadvantages of this system are that it exhibits considerable delay in steering, which results in poor controllability, and it has a runaway potential on downward grades unless excessive friction is designed into the drive mechanism.
The Beta method provides certain improvements over the Alpha method in respect to controllability. In accordance with the Beta method, the outside wheel is caused to increase in speed in order to effect turning. The inside wheel is allowed to coast to a lower speed at the same time. As compared with the Alpha method, the Beta method provides improved controllability, since the response to speed of the outside motor is forced, and therefore turning starts immediately. However, the Beta method of control is not entirely satisfactory, since the inside wheel will still coast to a lower speed at rates which are affected by the smoothness of the surface along which the vehicle is traveling. The Beta method remains subject to downgrade runaway possibilities and difficulties encountered in reducing the speed of the vehicle unless there is excessive friction in the system.
The prior art also discloses the use of dynamic braking techniques wherein the overrunning of a direct current drive motor is rapidly decelerated by causing the motor to act as a generator and converting the inertial energy to heat energy by shunting the armature of the motor across a braking resistor through the use of switching means (see U.S. Pat. Nos. 2,892,506 and 3,792,328). In accordance with the prior art teachings, the manual operation of a control member by a vehicle driver to a neutral position causes the switching means in the circuit to electrically connect the braking resistor across the motor in order to cause deceleration. This method offers some improvements in stopping and steering, but the vehicle movement is still characterized by an abruptness or jerkiness.
In accordance with this prior art dynamic braking technique, the braking function is only provided if the vehicle driver manually moves the control member to a zero speed, or neutral condition. Consequently, the primary attribute of this prior art technique is prevention of runaway on ramps and provision of quick stops when needed. However, deflection of the joystick from the zero speed or neutral position will release the dynamic brake and permit the vehicle to coast downhill at excessive speeds.
The Gamma method of control of the present invention departs from the foregoing prior art methods and techniques by simultaneously controlling the acceleration and deceleration of each of the drive motors to achieve the relative speed relationships necessary to the steering and speed conditions selected by operation of a manual control member by the vehicle driver. To that end, brake means operable to decelerate each of the drive wheels are provided and the control method operates to selectively drive, brake, or freewheel each of the drive wheels. Thus, turning is effected in the Gamma method by simultaneously driving the outside wheel to increase its speed and braking the inside wheel to decrease its speed.
In accordance with the Gamma method, an electrical control is provided. The control includes an electronic feedback loop for comparing electrical input control signals corresponding with desired drive wheel speeds with sensed output feedback signals corresponding with the actual speed of the drive wheels. The electrical control operates to retard the rotation of either drive motor whenever excessive output speed is sensed by the electrical control.
In the illustrated embodiment, the electrical control includes a closed-loop control circuit or servo system for each of the drive wheels. A braking resistor is arranged in the control circuit for dynamic braking in accordance with sensed operating conditions. The energization, consisting of applied voltage and/or back EMF of each drive motor, is monitored to provide a feedback signal which is compared with an electrical input control signal derived from a control member, such as a joystick manually operated by the driver of the vehicle. In accordance with this comparison, the voltage applied to the drive motor may be increased in order to accelerate the drive wheel, a braking switch may be operated to effect dynamic braking of the drive motor and deceleration of the drive wheel, or freewheeling of the drive motor may occur temporarily as the system moves between the acceleration and braking modes.
In contrast with prior art systems, the present invention provides simultaneous acceleration and deceleration of opposed or left and right drive wheels to effect positive and smooth steering. By virtue of both acceleration and deceleration, the vehicle control is not varied as a function of frictional ground conditions, as occurs in the prior art Alpha and Beta methods to effect slowing of the inside drive wheel.
The dynamic braking function per se, in accordance with the prior art teachings, is of limited value, since it is not automatic in that the braking function is dependent upon the vehicle driver's return of the control member or joystick to a zero speed or neutral position. In the Gamma method, the braking function is operable at a rate governed by the gain of the control system and braking can occur, therefore, tens, hundreds, or thousands of times per second. In stark contrast with the rate of braking in the prior art, the dynamic braking function in the Gamma method is more aptly referred to as "dynamic damping." This dramatic difference in the rate of brake application is perceivable by a vehicle occupant in terms of improved smoothness of operation and stability.
The closed-loop control of the Gamma method enables controlled speed during downgrade travel, which is not possible in the prior art dynamic braking systems or in the Alpha or Beta systems. The Gamma method also effects control of the smoothness of deceleration and motion reversal. With regard to the latter, the speed of operation of the drive motors can be significantly decreased before reversal is permitted.